Brolog as a EventEmitter

[huan]

For the maximum flexibility for the end-users for displaying the log messages, we plan to emit the log message as an event named loglevel.

For example:

log.on('verbose', (module, method, message) => {
  // filter module & method at here, and output whatever you want.
})

See: https://github.com/wechaty/wechaty-puppet-service/issues/152